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Proof step 4 from proof in (previous step)

perm(x, sort(x)), perm(sort(x), x) ⊦ a ∈ sort(x) ↔ a ∈ x

User applied rule case distinction

And closed the goal!

Rule argument

pos: right 1

Additional information

Used simplifier rules: in: ⊦ a ∈ y ∧ ¬ a ∈ x → ¬ perm(x, y); from specification list-perm